The invention relates to a method of manufacturing tubing and, more particularly, to an improved method and apparatus for increasing the inside or outside smoothness of cold drawn tubing that is drawn through a fixed die with an internal stationary mandrel plug disposed therewithin.
The cold drawing of tubes often involves the drawing of hot rolled tubular shells through a fixed die. The diameter and wall thickness of the tube shell are typically larger than the same dimensions of the finished tube. Simultaneous wall and diameter reductions may be achieved, moreover, by drawing the tube shell through a die opening, into which a cylindrical mandrel plug, fixed to a mandrel rod which holds it in a pre-set position, is inserted. The die opening, typically smaller than the outside diameter of the tube shell which is to be cold drawn, is generally provided with a bell shaped or conical shaped entrance or approach zone leading to a circular die land or bearing section. An annulus forms between the mandrel plug and the die opening. As the tube shell passes through the annulus its internal surface contacts a working surface on the mandrel plug and its external surface contacts the working surface of the die.
The outer diameter of the tube shell will be reduced to the diameter of the die land as it passes through the conical approach zone. The inner diameter of the tube shell is reduced simultaneously with its outer diameter, with little or no reduction of the wall thickness, until the inner surface of the tube shell contacts the cylindrical surface of the internally disposed mandrel within the conical approach zone. Thereafter, the inner diameter of the tube shell is essentially constant but the thickness of the wall decreases along with the outer diameter until entry into the die land. There is no further wall reduction after the shell enters into the die land. Excess metal due to the reduction in cross-section causes the length of the tube to increase. The cross section of the tube passing from the die opening is approximately equal to the cross section of the annulus within the die land.
The quality of the tube shell has considerable bearing upon the quality of the finished tube. It is essential, therefore, that the surfaces of the shell be free of defects such as seams and slivers to preclude the development of surface imperfections, pitting and unacceptable surface roughness in the drawn tube, particularly in tubular applications such as for hydraulic cylinders. Tube shells having defects may be properly conditioned by means of local or complete grinding of the surfaces of the shell. This necessitates additional labor intensive steps such as boring the inside surface of the shell or turning down the outside surface and results in the need to compensate for metal removed during boring or turning operations, or both.